Four-wire (4W) switched 56 Kbps service affords a synchronous 56kilobit per second (Kbps) connection to a switched digital communication network from Customer Premises Equipment (CPE). This network consists of channels in which voice and/or data information is transmitted in the form of time division multiplexed signals at the rate of 64 Kbps. The information for each channel is divided up into bytes/words consisting of eight bits of information.
The bytes are, for example, multiplexed into frames of twenty-four eight-bit bytes over a T1 carrier having 24 trunk lines. The digital signal is multiplexed as one byte per trunk, plus a framing bit for each frame of the digital signal. The framing bit is used to identify where the byte allocated to each of the 24 trunks are located on the T1 carrier. The framing bit has a pattern which takes twelve frames to repeat. These twelve frames, numbered one through twelve, are termed a superframe.
Normally all eight bits of each byte transmitted during a frame contain encoded voice or digital data information. However, in every sixth frame the least significant bit is replaced with a signalling bit by, for example, the digital switch. These signalling bits are used between the digital switch and the CPE to indicate to the CPE the progress of communication during a call setup period.
Thus, the sixth and twelfth frames for each superframe include a signalling bit in the eight bit position of each of the twenty-four bytes. Within this network, the presence of the byte boundaries yields the capability to send and receive eight-bit bytes containing digitized representations of dual tone multi-frequency (DTMF) address signaling as well as voice and 56 Kbps data.
Although the network depends on aligned bytes for DTMF signalling, the 4W switched 56 CPE only transmits a synchronous bit stream without a byte alignment. The lack of byte boundary information at the switched 56 Kbps connection to the network has precluded the sending of DTMF addressing (dialing) signals. As such, all four-wire switched 56 Kbps connections are currently using dial pulse signaling.